During continuous operation of the wave soldering machine, solder dross is continuously generated by the combined effects of thermal oxidation of molten solder and Flux carbonization. These grayish-black floating dross not only cover the surface of the molten solder to reduce heat transfer efficiency, but are also repeatedly drawn into the wave nozzle under pump agitation, causing wave profile distortion and sudden fluctuations in liquid level height. Our on-site measurements at a customer site in Dongguan found that when the accumulated solder dross thickness exceeds 5mm, the standard deviation of wave height surges from 0.12mm to 0.38mm, directly leading to local insufficient solder or solder dragging defects when PCBs pass through the wave.

Dross skimming may seem simple, but the timing and technique directly affect the stability of the subsequent two hours. The JJY SMT workshop implements a manual dross skimming system with intervals of no more than 4 hours per shift, using a high-temperature resistant stainless steel mesh ladle to slowly push flat along the furnace side wall, avoiding agitation of the deep molten solder to prevent accelerated oxidation. It is also required that the distance between the mesh ladle and the liquid surface is always controlled within 2cm, which can effectively trap floating dross without bringing out excessive liquid solder. The measured single skimming amount is maintained between 80g and 120g, corresponding to a safe range where the dross layer thickness falls back to 2.5mm to 3.0mm.

It is worth noting that the dross skimming frequency cannot be mechanically applied, and must be comprehensively judged based on the solder mask type of incoming PCBs, component placement density, and flux solid content on the day. For example, when using high-solid-content rosin-based flux, the solder dross generation rate is 1.7 times faster than that of water-based flux, in which case the interval needs to be shortened to 2.5 hours. After large batches of small-pad boards pass through the wave, the dross amount is significantly higher than that of large-pad boards, which is related to the more sufficient thermal decomposition of flux residues between densely arranged pads.

We once delayed dross skimming for 6 hours due to misjudgment of the dross layer state, and as a result, consecutive pin bridging occurred on 3 QFP devices on the second test board. Under the microscope, solder dross particles were found embedded in the top of the wave to form tiny protrusions, causing abnormal accumulation of solder flow in the gaps between pins. After rework, we recalibrated the dross skimming rhythm and installed a liquid level viewing window, and no similar defects occurred in the following 72 consecutive hours, indicating that solder dross management is not an auxiliary process but a core control point in the wave soldering process.

After dross skimming, the molten solder surface should present a uniform metallic luster without large dark spots. If local graying or aggregation of floating black spots is found, it indicates that the fluidity of the solder in this area has decreased, and it is necessary to simultaneously check whether the nozzle solder baffle is deformed and whether the pump speed deviates from the set range of 1200rpm±20rpm, because solder dross itself does not cause defects alone; it is only the most intuitive indicator signal of system imbalance.